RIASSUNTO
Summary
The reuse of existing well bores is a cost-saving technique used by oil and gas producers in mature fields. This process usually requires the removal of tubing and packers to allow the well to be deepened or sidetracked. The tubing removal process is often made difficult when the tubing is stuck inside the casing because of mechanical binding or binding caused by dehydrated mud or sand in the annulus between the tubing and casing. Conventional direct and indirect methods of finding the stuck point(s) along an interval of tubing, such as free-point indicator tools or acoustic attenuation measurements, have proved themselves useful in finding the point(s) where the tubing is stuck. However, when the sticking is caused by uncompacted sand that has entered the annular space between the tubing and casing, these conventional stuck-pipe indication methods are often inconclusive or misleading.
Technological advancements in the design and development of slim, 1 11;'16-in. outside diameter (OD), radial cement bond tools allow the application of a new measurement to find sand-stuck tubing intervals. These tools provide up to six independent, closely spaced acoustic attenuation measurements distributed radially around the tool body. When the recording operation is performed inside the tubing, the increased sensitivity of these tools to sound attenuation, as compared with conventional omnidirectional amplitude signals, allows sand intervals along the outside of the tubing to be distinguished from intervals where little or no sand is present. This information is used to select the best depth to sever or back-off the free tubing in order to reduce the interval length and expense associated with the washover process required to remove the remaining tubing from the wellbore.
An overview of conventional stuck-pipe recovery techniques is provided here, along with a brief explanation of applicable acoustic technology. A case study of a South Louisiana well is presented to demonstrate the application and illustrate the cost savings which can be gained by use of this very new technology.
Introduction
Oil and gas producers operating in maturing fields such as the U.S. Gulf of Mexico basin continually strive to reduce the expense of drilling new wells. The presence of oyster leases and dredging restrictions in inland bay fields severely limits, and in some cases prohibits, the drilling of new wells. One method of overcoming these obstacles includes reentering existing wells to take advantage of the casing string already cemented in place. This process usually requires the removal of old tubing and packer assemblies that were once used to produce hydrocarbons from now-depleted zones. Their removal clears the way for recompleting to shallower zones, deepening the well, or sidetracking to reach new reservoirs.
The recovery of stuck tubing has plagued the oil and gas industry since its inception and is commonly necessary in remedial well operations (Bernat 2001). Conditions that can cause tubing to get stuck are often mechanical in nature, including collapsed or parted casing, dehydrated drilling mud, and formation sand filling the annular space between the tubing and casing.
The most common methods of recovering stuck tubing include impact tools (jars), washover operations, low-frequency vibrators, and milling tools (Stoesz and DeGeare 2000). These recovery techniques have been used in the industry for many years with varying degrees of success, depending upon the amount of time spent on the process. Because jarring, washover, and milling techniques require a drilling or workover rig to be moved to the wellsite, the amount of time spent on the pipe recovery process is kept to a minimum so as to avoid a negative impact on project economics.
Risk factors such as the type of sticking, hole angle, well depth, and material to be removed should be evaluated to determine the best method of pipe recovery (Stoesz and DeGeare 2000). The usual process for removing mud-stuck or sand-stuck tubing is to reciprocate the tubing with the rig to free up a length of tubing, and then to perform an electric line backoff to allow recovery of the free section. Wash pipe is used to circulate and remove dehydrated mud or sand by washing over the remaining tubing string, and then the free-point, backoff, and recovery process is repeated (Walker 1984). When the tubing cannot be freed by jarring and reciprocating, the challenge is to determine the proper depth interval for washing over the tubing in order to minimize rig time and other associated costs.